This invention relates to instrument systems, such as are used for measurement and control of industrial processes. More particularly, this invention relates to compensation of errors in electronic instrument systems in which a physical variable (such as the level of material in a vessel) is converted by a sensor (sometimes referred to as a "probe") to an electrical signal (such as a capacitance, admittance, or a current which is related thereto) which is presented as an input to an instrument system. Still more particularly, this invention relates to compensation for errors which may arise in such systems, by selectively coupling different inputs to a common signal processor, and providing a compensated output based on the responses of the signal processor to the different inputs. This invention also relates to systems for communicating an output, such as the compensated output, to a remote location and receiving instrument instructions from a remote location.
Instrument systems are subject to errors arising from a number of sources. An example of such an instrument system is a radio frequency admittance responsive system, for instance as shown in U.S. Pat. No. 1,416,834. Such a system may be used to measure the level of materials in a vessel; a vertically disposed sensor provides an admittance which is a function of material level, and an admittance-responsive instrument coupled to the sensor provides an output which is a function of the sensor admittance. Errors, i.e. deviation of the instrument output from the value corresponding exactly to the physical variable sought to be measured, can arise from a number of sources. One source is the transfer function of the sensor itself. For instance, the output of a vertically disposed sensor admittance may be a function of material electrical properties (e.g. dielectric constant) as well as the material level. In the past, such errors have been compensated by providing an additional separate instrument system which responds to material electrical properties but not to material level, and using the output of the separate instrument system as a reference to compensate the primary instrument output for changes in material properties. See, e.g., U.S. Pat. No. 4,232,300. Such systems are generally expensive and complicated.
Another source of error in such systems is the transfer function of the instrument. The transfer function may change for instance due to aging of components, environmental influences such as changes in temperature of components, or changes in measuring conditions such as instrument loading. Such errors may appear as offset or "zero" errors, which are independent of the measured value, and/or gain or "span" errors which are a function of the measured value. In the past, such error sources have typically been addressed by "brute force" methods such as use of stable precision components and circuit designs in which the effects of environmental and measuring conditions on circuit operation are minimized. Systems employing such methods are also generally expensive and complicated.
The foregoing problems, and the drawbacks of prior art attempts to address them, tend to be particularly acute in radio frequency admittance monitoring systems, especially loop powered or "two-wire" instrument systems where the operating power for the instrument system is severely limited. For instance, radio frequency admittance monitoring systems are generally required to have high isolation between their sensor admittance-measuring circuitry and their output circuitry in order to prevent an explosion hazard resulting from introduction of high energy electrical potentials into a flammable atmosphere which may surround the sensor. The complexity and expense of providing precision measurements by brute force methods while also providing such isolation are substantial, particularly in systems which process signals from several sensors.
In radio frequency admittance monitoring instruments of the "two wire" type, i.e. in which the instrument receives its operating power from and transmits its output signal to a single pair of signal wires connecting the instrument to a remote location, improving precision of measurement is particularly problematic because of the constraints on power available to the instrument. Two wire instruments are generally required to provide an output signal in the range of 4-20 mA, which means that the instrument must operate under worst-case conditions on a current budget of less than 4 mA. With as little as 10 V available from the signal wires at the instrument, this requires the instrument to be able to operate with a power less than 40 mW. To improve the precision of measurement in such an environment by conventional means generally requires use of more power; for instance, the amplitude stability of an RF oscillator and the gain stability of an RF amplifier generally degrade rapidly with decreasing power.
Radio frequency two-wire admittance monitoring systems have heretofore been limited in their modes of communication with equipment in a remote location. They have generally provided an analog output signalling current varying from 4 mA to 20 mA in response to changes in the measured admittance; some have provided a digital output signal in a limited nonstandard format. They have also been limited in their ability to receive signals, such as signals representing instructions to the instrument, from the signal wires.